The present invention relates generally to magnetic disk drives and, more particularly, to a disk drive with an adaptive control path that produces an adaptive control effort based on a characterized deviation between actual plant frequency response and modeled plant frequency response.
Magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer that is located at the far end of a moveable actuator. A servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably move the transducer from track to track (xe2x80x9cseekingxe2x80x9d) and to hold the transducer at a desired position (xe2x80x9ctrack followingxe2x80x9d). A detailed discussion of servo control systems is unnecessary because such systems are well known as set forth, for example, in patent application Ser. No. 09/138,841 that was filed on Aug. 24, 1998, entitled xe2x80x9cDISK DRIVE CAPABLE OF AUTONOMOUSLY EVALUATING AND ADAPTING THE FREQUENCY RESPONSE OF ITS SERVO CONTROL SYSTEM,xe2x80x9d and is commonly owned by the assignee of this application.
Most disk drives have previously been used for storing conventional data files of the type that are associated with personal computers. In such applications, data integrity is paramount relative to other considerations such as seek times and the reduction of acoustic noise. It presently appears, however, that disk drives are likely to become popular for recording and replaying audiovisual dataxe2x80x94e.g. a drive based recording device that replaces video cassette recorders (VCRs). A drive-based recording device of this nature will benefit from using a disk drive with faster seek times because it will spend less time moving its actuator where it needs to be and have more time to record or recover information in any given unit of time. The drive-based recording device, therefore, may be able to record and/or playback more audiovisual data streams that otherwise possible. At the same time, the drive-based recording device is likely to be located adjacent to a television or be in some other location where acoustic noise issues arise. Accordingly, it is equally important for the disk drive to implement its seeks in as quiet a manner as possible.
The drive industry has progressed through several stages of development as related to seeks. Of particular relevance here, is the prior use of a so-called xe2x80x9cbang-bangxe2x80x9d seek profile wherein the transducer is rapidly accelerated at the start of a seek and then rapidly decelerated at the end of a seek using saturated levels of command effort (current or voltage). A bang-bang seek profile moves the transducer to a target position in as rapid a manner as possible. On the other hand, because the bang-bang acceleration profile is a square wave, it is relatively loud and it contains many high frequency components that may detrimentally excite a mechanical resonance that causes the transducer to take longer to settle into the target position. It has previously been determined that a quieter, faster settling seek is possible by xe2x80x9cshapingxe2x80x9d the transducer""s acceleration profile so that it does not appear like a square wave, but rather approximates a single frequency sine wave. The result is a shaped seek profile that is xe2x80x9cclosexe2x80x9d to a bang-bang square wave but is quieter and does not contain the high frequency components that may excite the drive""s resonant frequencies.
Modern disk drives generally use a sampled servo control system that only periodically receives position information (e.g. once per servo sector) and shortly thereafter outputs a command effort based on a deviation between the indicated position and the target position. The servo control system in such a drive implements a shaped seek profile as a feed-forward profile using a feed-forward control path as a feed-forward profile using as discussed, for example, in co-pending patent application no. (pending) that was filed on Mar. 31, 2000, entitled xe2x80x9cDISK DRIVE WITH FEED-FORWARD CONTROL PATH THAT RECEIVES A REFERENCE POSITION SIGNAL TO APPLY A FEED-FORWARD COMMAND EFFORT AT A RATE GREATER THAN A SERVO SAMPLING RATE,xe2x80x9d (xe2x80x9cthe Multi-rate Feed-Forward Applicationxe2x80x9d), which application is commonly owned by the assignee of this application and hereby incorporated by reference in its entirety.
Modeling errors in the feed-forward control path, however, may cause the servo to inaccurately follow the intended feed-forward profile. In the referenced co-pending application, the feed-forward control path does not model the plant at all but rather implements a simple double derivative of the reference position signal to form a sinusoidal seek profile without regard to the frequency-dependent variance of the plant. The benefits of the multi-rate feed-forward path in the foregoing application, moreover, diminishes with the number of servo samples such that it only provides a practical benefit for short to medium seeks.
The most prevalent modeling errors are gain errors due to variation in the values of the motor torque constant (KT) and the motor winding resistance (RW) due to changes in temperature, calibration errors, track pitch errors, and other factors. These modeling errors may cause the actuator to either overshoot or undershoot the target position at the end of the shaped seek, and thereby increase the required settling time for seeks of all lengths.
One could simply rely on the feedback loop that contains the feedback control path to repeatedly, reactively reign in any deviation caused by the feed-forward control path""s modeling error in a sample by sample fashion (as in the above-referenced Multi-rate Feed-forward application), but system performance would be improved if the effect of the modeling error could be eliminated or reduced.
There remains a need, therefore, for a disk drive with a servo controller which provides improved performance by reducing or eliminating variance due to modeling error in a feed-forward control path.
The invention may be regarded as a disk drive comprising a plant and a servo controller with an adaptive control path that produces an adaptive control effort based on a characterized deviation between an actual plant frequency response (G) and a modeled plant frequency response (G0). The plant has an actual frequency response (G) and the servo controller includes a feed-forward control path that operates according to a modeled frequency response (G0) of the plant. The plant more specifically comprises a transducer that repetitively samples position information during sample periods to produce a signal representing an indicated position (y), and a voice coil motor (VCM) adapted for positioning the transducer in response to a total command effort signal (u), The servo controller generates the total command effort signal (u) for moving the transducer from a start position to a target position. The servo controller comprises (a) a reference position generator that produces a reference position signal (r) that varies as a function of time and represents a shaped position profile to be followed by the transducer as it moves from the start position to the target position; (b) a feed-forward control path that operates according to a modeled frequency response (G0) of the plant, the feed-forward control path receiving the reference position signal (r) and producing a feed-forward command effort signal (uffwd) that corresponds to moving the transducer along the shaped position profile when the actual frequency response (G) is equal to the modeled frequency response (G0); (c) a feedback control path including a difference junction that differences the reference position signal (r) and the indicated position signal (y) to produce an error signal (e) when the transducer does not move along the shaped position profile due to a deviation between the actual frequency response (G) and the modeled frequency response (G0), the feedback loop producing a feedback command effort signal (ufb) based on the error signal (e); (d) an adaptive control path that (i) receives the error signal (e) over a plurality of initial sample periods to characterize the deviation between the actual frequency response (G) and the modeled frequency response (G0), (ii) and then produces an adaptive command effort signal (uadapt) based on the characterized deviation to cause the transducer to more closely follows the shaped position profile as it continues to the target position; and (e) a summing junction that combines the feed-forward command effort signal (uffwd), the feedback command effort signal (ufb), and the adaptive command effort signal (uadapt) to produce the total command effort signal (u) that is provided to the VCM.